Hearing loss is an important problem that affects the quality of life of millions of people. About 15% of American adults (37.5 million) reports problems with hearing. For most cases, the problem relates to frequency-dependent loss of sensitivity of hearing. In FIG. 1, the bottom (dashed) curve corresponds to the Absolute Hearing Threshold (AHT) as a function of frequency. The AHT is the sound level that is almost audible for normal hearing subjects. The top (dash-dotted) curve represents the Uncomfortable Loudness Level (UCL) for the average normal hearing population. Generally speaking, human sensitivity to acoustic inputs deteriorates with age. The raised hearing threshold for a particular person may be represented by the middle (solid) curve in FIG. 1. Now consider an ambient tone at intensity level L1 as indicated by the black circle. This signal would be heard by a normal listener but not by the impaired listener. The primary task of a hearing aid is to amplify the signal so as to restore normal hearing levels for the “aided” impaired listener. Aside from signal processing that compensates for problems that occur due to insertion of the hearing aid itself (e.g., feedback, occlusion, loss of localization), an important challenge in hearing aid signal processing design is to determine the optimal amplification gain L2−L1.
Technically, the optimal gain depends on the specific hearing loss of the user and turns out to be both frequency and intensity-level dependent. In commercial hearing aids, amplification is generally based on multi-channel dynamic range compression (DRC) processing in the frequency bands of a filter bank. A typical gain vs. signal level relation in one frequency band of a DRC circuit is shown in FIG. 2. The gain is maximal for low input levels and remains constant with growing input levels until a Compression Threshold (CT), after which the logarithmic gain decreases linearly (in dB). The slope of the gain decrease is determined by the compression ratio CRΔinput/Δ(input+gain), which is a characteristic parameter for DRC algorithms. Aside from CT and CR, a DRC circuit is typically also parameterized by attack and release time constants (AT and RT, respectively) to control the dynamic behaviour. The crucial problem of estimating good values for the parameters CT, CR, AT and RT is an important part of the so-called fitting problem.
Today's hearing aids are usually provided with a hearing loss signal processor and a number of different signal processing algorithms including DRC. Typically, each of the signal processing algorithms is tailored to particular user preferences and particular categories of sound environment. Initial signal processing parameters of the various signal processing algorithms including CT, CR, AT, and AR, are determined during an initial fitting session in a dispenser's office and programmed into the hearing aid by activating desired algorithms and setting algorithm parameters in a non-volatile memory area of the hearing aid in question.
Modern hearing aid fitting strategies set compression ratios by prescriptive rules, e.g., the NAL rules, see D. Byrne, H. Dillon, T. Ching, R. Katsch, and G. Keidser, “NALNL1 procedure for fitting nonlinear hearing aids: Characteristics and comparisons with other procedures,” Journal of the American Academy of Audiology, vol. 12, no. 1, pp. 37-51, January 2001, and DSL rules, see L. E. Cornelisse, R. C. Seewald, and D. G. Jamieson, “The input/output formula: a theoretical approach to the fitting of personal amplification devices,” The Journal of the Acoustical Society of America, vol. 97, no. 3, pp. 1854-1864, March 1995, are very widely used. For the dynamic parameters AT and RT no standard fitting rules exist and most hearing aid manufacturers offer slight variations on known dynamic recipes such as slow-acting (‘automatic volume control’) and fast-acting (‘syllabic’) compression.
The goal of determining hearing aid signal processing parameters, such as CT, CR, AT, RT, utilizing prescriptive fitting rules is to provide a decent ‘first-fit’ of the hearing aid in question. Typically, an audiologist spends a very limited amount of time on fitting a hearing aid to each user compared to all the nuances that are associated with hearing loss. Diagnostic procedures exist which would optimize the prescribed hearing aid parameters to maximize the benefits that the user would get out of their hearing aids. Unfortunately, the time needed to carry out these procedures is prohibitive for the audiologist and instead they often resort to an automatic fitting procedure with minimal personalization. This may result in several return visits to the audiologist for the user, and too often, the user gives up and deems the hearing aid as being more of a burden than a benefit and the hearing aid ends up not being used.
Another fundamental challenge is that the user typically experiences unforeseen and changing sound environments that were not taken into account when the hearing aid was fitted to the user.